Add AddCharVariable() for creating NC_CHAR array variables

[juselius]

SDSlite currently multiplexes the ubyte type for char and ubyte when reading and writing variable data. However, it's currently not possible to create a variable of type char using AddVariable<>(). The problem is that the .NET char type is 16-bits, whereas the NC type is 8-bits. When creating a variable of type byte, it always becomes nc_ubyte. This patch adds a new specialized method AddCharVariable() for NC DataSets which creates char typed variables. Not very nice, but the best quick fix I could come up with.